The present invention generally relates to semiconductor optical devices and more particularly to a semiconductor optical device having a non-linear operational characteristic.
Conventionally, a so-called non-linear semiconductor optical device is studied in relation to the optically operated optical switches, optical bistable devices, optically addressed optical memories and the like. The principle of such a device is based on the switching of its state between an operational state and a non-operational state triggered by an irradiation or alternatively by an interruption of a control optical beam such that the transmission of an optical beam or information optical beam is controlled in response to the control optical beam. For example, the device allows transmission of the information optical beam in the operational state and prohibits the transmission of the information optical beam in the non-operational state, or vice versa.
In such semiconductor optical devices, it is required that the device has an excellent high frequency response particularly when the device is to be used in the optical computers and the like for the optical logic operations. Further, it is desired to control the length of the duration in which the device is in the non-operational state, as desired.
Conventionally, such a non-linear semiconductor optical device is constructed as illustrated in FIG.1, wherein the device represented by M has a superlattice structure based on a repetition of a structural unit which comprises a first material layer Ml of gallium arsenide (GaAs) having a small band gap and a pair of second material layers M2 of aluminum gallium arsenide (AlGaAs) having a large band gap disposed at both sides of the first material layer M1.
FIG.2 shows the energy band structure of this prior art device. In the drawing, the Fermi level is represented by E.sub.F, the valence band is represented by Ev, and the conduction band is represented by Ec. Note that the energy gap or band gap between the conduction band Ec and the valence band Ev is substantially larger in the second material layer M2 than in the first material layer M1. Such a first material layer M1 sandwiched between the second material layers M2 forms a band structure known as quantum well in which the energy level of the electrons .DELTA.En measured from the bottom edge of the conduction band Ec of the first material layer M1 is given approximately as EQU .DELTA.En=(h.sup.2 /8m).(n/L).sup.2 ( 1)
where n stands for an integer, m stands for the effective mass of an electron in the first material layer M1, h stands for the Planck's constant, and L stands for a thickness of the first material layer M1. Note that the energy level of electrons in the quantum well depends on the effective mass m of the electron and the thickness L of the first material layer M1.
In such a structure, it is known that there is formed an exciton that is an elementary excitation formed by an electron and a hole bounded to each other by the Coulomb interaction, at an energy level slightly lower than the aforementioned energy level by an amount EQU 8.mu.e.sup.4 .pi..sup.2 /(k.sup.2 h.sup.2) (2)
where .mu. stands for the reduced mass of an electron and a hole, k stands for the dielectric constant of the first material, h stands for the Planck's constant and e stands for the elementary electric charge.
In operation, the wavelength of the information optical beam to be controlled by the device is chosen such that the beam causes a resonance with the excitons existing in the layer M1, and the wavelength of the control optical beam for triggering the switching of the device is set such that the control optical beam excites the free electrons to an energy level close to the energy level of the excitons already existing in the layer M1 or such that the control optical beam excites the excitons that have the foregoing energy level.
Next, the principle of operation of this prior art device will be described with reference to FIG.1. When a control optical beam I.sub.2 is irradiated to the device under a state that the device is irradiated further by an information optical beam I.sub.1 as illustrated in FIG.1, the energy transferred to the device from the optical beam I.sub.2 causes an excitation of the free electrons or excitons as already described. The free electrons thus excited causes a screening of the Coulomb interaction, and the screening thus caused disturbs the formation of the excitons in turn. The excitation of excitons by the control optical beam also disturbs the formation of the excitons by causing a releasing of the free electron from the excitons. When such an impediment against the formation of the excitons occurs, the absorption of the information optical beam I.sub.1 by the excitons in the layer M1 is reduced. This means that the transparency of the device is creased accordingly. The transition from the operational state to the non-operational state is extremely fast and can be achieved in a matter of about 500 femtoseconds. Thus, the optically controlled switching of such a non-linear optical semiconductor device provides a possibility of optically operated logic devices such as optical switches, optical bistable devices, optically addressed memories and the like.
Such an optical control of the transmittance should be observed also in a bulk crystal of gallium arsenide and the like. However, such a bulk crystal contains therein significant thermal excitations at the room temperature, that such thermal excitations obscure the observation of excitons. Thus, the construction of the non-linear optical semiconductor devices utilizing the optical interaction of light with excitons based on the bulk crystal is extremely difficult. By designing the device to have the superlattice structure that confines the excitons two-dimensionally, the excitons become observable clearly and thus the non-linear semiconductor optical device for the first time becomes the matter of reality.
In the buil crystal, the diameter of a free exciton at the 1S state is represented as EQU r=h.sup.2 .epsilon./(2.pi..sup.2 e.sup.2 .mu.) (3)
where .mu. stands for the reduced mass of an electron and a hole, .epsilon. stands for the dielectric constant, h is the Planck's constant and e stands for the elementary electric charge. Thus, the thickness of the first layer M1 has to be substantially equal to or smaller than the diameter r given by Eq.(3). When gallium arsenide is used for the first layer M1, r is about 280 .ANG..
FIG.3 shows another prior art non-linear optical semiconductor device comprising the superlattice structure body M already described with reference to FIG.1 wherein there is further provided a pair of mirrors E at both ends of the body M to form a Fabry-Perot resonator. The mirror E may be a dielectric multilayer mirror comprising a number of silicon oxide layers and aluminum oxide layers stacked alternately. As the super lattice structure body M used in the device of FIG.3 is identical with the superlattice structure M in FIG.1, the band structure for the device of FIG.3 is identical with the structure shown in FIG.2.
The device of FIG.3 is constructed such that, when the information optical beam I.sub.1 is irradiated on the device under a state that the control optical beam I.sub.2 is not incident thereupon, there holds a relationship EQU m.lambda./2=nL
where m is an integer, .lambda. stands for the wavelength of the optical beam I.sub.1, L stands for the thickness of the superlattice structure M, and n stands for the refractive index of the superlattice structure M. Note that the foregoing relationship indicates the resonance, and the thickness L and the refractive index n of the device is set such that there holds a resonance when the device is irradiated only by the information optical beam I.sub.1.
When the above condition is met, the absorption of the information optical beam I.sub.1 by the device becomes minimum and the optical beam I.sub.1 passes through the device without substantial absorption. When, on the other hand, that the device is further irradiated by the control optical beam I.sub.2, the free electrons causing the screening of the Coulomb interaction are excited or the excitation of excitons having energy level corresponding to the control optical beam I.sub.2 is caused. As a result, formation of the excitons interacting with the information optical beam I.sub.1 is disturbed or reduced and the refractive index n of the superlattice structure M is changed. When the refractive index n is changed, the foregoing relation corresponding to the resonance does not hold anymore. This state is illustrated in FIG.4. Note that the reflection of the optical beam I.sub.1 by the mirror E does not occur at the node of the optical beam because of the change of the effective optical path between the pair of mirrors E. As a result,the absorption to the information optical beam I.sub.1 is increased and the transmission of the beam I.sub.1 through the device is interrupted. Thus, this device of FIG.3, too, controls the transmission of the information optical beam in response to the irradiation and interruption of the control optical beam I.sub.2.
In addition to the aforementioned devices, there are a number of possibilities to construct such a non-linear semiconductor optical device wherein the control of the transmission of optical beam is achieved by irradiation of another optical beam. Some typical examples are as follows.
a) A device constructed such that it is opaque when the control optical beam is not irradiated but becomes transparent when the control optical beam is irradiated. In such a device, the resonance appears only when the control optical beam is irradiated additionally to the information optical beam.
b) A device of which state is changed between a completely off-resonant state ((m+1/2).lambda./2=nL) and an incompletely off-resonant state. This device is further divided into a first type in which the completely off-resonant state is achieved in response to the irradiation of the control optical beam and thereby causing a transition from a transparent state to an opaque state, and a second type in which the incompletely off-resonant state is achieved responsive to irradiation of the control optical beam and thereby causing transition from the opaque state to the transparent state.
c) A device of which state is changed between a first off-resonant state close to the resonant state and a second off-resonant state that is close to the completely off-resonant state.
In any of these prior art devices, there is a problem in that the recovery from the operational state to the non-operational state needs a duration that is substantially longer than the time for the transition from the non-operational state to the operational state. FIG.5 shows a response of the device obtained when a control optical beam is irradiated on the device of FIG.1 in the form of an optical pulse with a pulse width of 100 femtosecond. As can be seen from the plot, the time needed to cause transition from the operational state to the non-operational state is only about 0.4 picoseconds while the time needed to recover the original operational state is much longer. The reason of this slow recovery is attributed to the long relaxation time for removing or expelling the free electrons from the first layer M1 after the termination of the control optical beam. Such a slow removal of the electrons deteriorates the high frequency characteristic of the non-linear optical semiconductor device significantly and prohibits the repetitive operation of the device at high speed.
FIG.6 shows the response for the case of the device of FIG.3. In this device, too, the recovery of the operational state is very slow or even worse than the response of FIG.5. As long as the non-linear semiconductor optical device has such a slow response for the recovery to the operational state, the device cannot be used for the high speed optical logic devices of future optical computers.